The word memristor is a portmanteau of memory and resistor where the state of the device has a memory property, achieved by a historical voltage/current applied to the device. This type of device was theorized in 1971 by L. Chua. Around 2008, Hewlett Packard used oxygen ion migration through the bulk crystalline structure of titanium dioxide, bounded by platinum electrodes. Various attempts at a memristor implementation have had the challenge of requiring very high fields to force ion migration through the bulk, leading to formation of conductive internal paths, which suffers from instabilities in, for instance, the TiO2 crystal structure.